The invention relates to an objective with mirrors whose central mirror apertures cause a pupil obscuration. An objective of the type considered herein comprises two partial objectives, the first partial objective projecting a first field plane onto an intermediate image, and the second partial objective projecting the intermediate image onto a second field plane. Such objectives are used, for example, as projection objectives in microlithography or as inspection objectives for observing surfaces, in particular wafer surfaces.
Catoptric reduction objectives with a pupil obscuration and intermediate image for application in microlithographic projection exposure apparatus are disclosed in EP 0 267 766 A2. The exemplary embodiments shown in FIG. 2 and FIG. 3 of EP 0 267 766 A2 represent objectives with a first partial objective and a second partial objective. The two partial objectives in the case of FIGS. 2 and 3 constitute two mutually opposing quasi-Schwarzschild objectives with different magnification ratios. The quasi-Schwarzschild objectives are constructed from a convex and a concave mirror which in each case have a central mirror aperture. In the case of the objectives shown there, the aperture obscuration of 0.38 or 0.33 is relatively large by comparison with the image-side numerical aperture of 0.3. Moreover, the objectives have only a magnification ratio of 0.6 or of 0.4. The numerical aperture at the intermediate image is greater than in the image plane due to the configuration of the two mutually opposing quasi-Schwarzschild objectives.
A reflective projection objective for EUV (Extreme Ultraviolet) lithography with pupil obscuration, but without an intermediate image, is disclosed in U.S. Pat. No. 5,212,588. The projection objective comprises a convex mirror with a central mirror aperture, and a concave mirror with a central mirror aperture. The rays emanating from the object plane are reflected four times at the two mirrors before they strike the image plane. The image-side numerical aperture is only between 0.08 and. 0.3 in the case of an aperture obscuration of between 0.4 and 0.7. The magnification ratio in the exemplary embodiments is between −0.3 and −0.2.
A further reflective projection objective for EUV lithography with pupil obscuration, but without an intermediate image, is disclosed in U.S. Pat. No. 5,003,567. In this case, the projection objective comprises a pair of spherical mirrors which are coated with multilayers and have a common center of curvature. In this case, the first mirror is a convex mirror, while the second mirror is a concave mirror. However, these objectives of the Schwarzschild type have a large image field curvature, and U.S. Pat. No. 5,003,567 therefore proposes applying the structure-carrying mask (reticle) to a curved substrate.
Reflective projection objectives for EUV lithography with pupil obscuration and intermediate image are also disclosed in EP 1 093 021 A2. The first partial objective, arranged between the object plane and the intermediate image, has four or six mirrors which are inserted extra-axially except for the mirror arranged in the aperture plane. The first partial objective does not lead to pupil obscuration in this case. The second partial objective comprises a convex mirror with an extra-axial mirror aperture, and a concave mirror with an extra-axial mirror aperture. The mirror which lies geometrically closest to the image plane is a convex mirror, and accordingly the thickness of the mirror substrate is greatest on the optical axis. This leads to a greater aperture obscuration when the free image-side working distance and the substrate thickness of the convex mirror are considered. In addition, convex mirrors generally have a lesser diameter than concave mirrors, since they have a diverging optical power. However, in the case of a lesser mirror diameter, the mirror obscuration, that is to say the ratio of the diameter of the mirror aperture to the diameter of the mirror, is more unfavorable.
A catoptric microscope objective with pupil obscuration, but without an intermediate image, is disclosed in U.S. Pat. No. 4,863,253. It comprises a convex mirror without a central mirror aperture, and a concave mirror with a central mirror aperture. In this arrangement, after reflection at the concave mirror the rays do not pass through a mirror aperture in the convex mirror, but are guided past the first mirror on the outside. This leads to a very high aperture obscuration by the convex mirror.
The publication entitled “Aplanatic corrector designs for the extremely large telescope” by Gil Moretto (Applied Optics; Vol. 39, No. 16; 1 Jun. 2000; 2805-2812) discloses a mirror telescope which has, downstream of a spherical primary mirror, a correction objective which corrects the spherical aberration and coma caused by the primary mirror. In this case, the correction objective projects the intermediate image formed by the primary mirror onto the image plane of the telescope enlarged with a magnification ratio of 3.5. The objective comprises two concave mirrors which project the intermediate image onto a further intermediate image, and a pair of mirrors composed of a concave mirror and a convex mirror which project the further intermediate image onto the image plane of the telescope. The projection of the intermediate image formed by the primary mirror onto the further intermediate image has a reduction ratio of −0.9, while the projection of the further intermediate image onto the image plane of the telescope is enlarged with a magnification ratio of −3.75. The numerical aperture is 0.1 at the image plane of the telescope and 0.345 at the intermediate image. Because of the mirror apertures, the objective has a pupil obscuration which is relatively large by comparison with the numerical aperture. The correction objective also has a relatively large field curvature, since the convex mirror has only a slight curvature.
A correction objective for a telescope is also disclosed in the publication entitled “Optical design of the Hobby-Eberly Telescope Four Mirror Spherical Aberration Corrector” by R. K. Jungquist (SPIE Vol. 3779, 2-16, Jul. 1999). The optical design is very similar to the previously described correction objective. It is exclusively concave mirrors that are used in the correction objective shown, and so the field curvature is relatively large.
Controllable micromirror arrays are disclosed in the publication entitled “Digital Micromirror Array for Projection TV” by M. A. Mignard (Solid State Technology, July 1994, pp. 63-68). Their use as object to be projected in projection exposure apparatus forms the content of U.S. Pat. No. 5,523,193, U.S. Pat. No. 5,691,541, U.S. Pat. No. 6,060,224 and U.S. Pat. No. 5,870,176. In the exemplary embodiments described there, the respective projection objective is, however, illustrated only diagrammatically. Concrete exemplary embodiments for projection objectives which are adapted to the requirements of so-called maskless lithography are not contained in the patents.
A catadioptric projection objective with pupil obscuration and intermediate image is disclosed in DE 197 31 291 C2. In this case, the objective has a refractive and a catadioptric partial objective, and is used in a wide UV wavelength region. In addition to lenses for color correction, a concave mirror and an approximately plane mirror are arranged in the catadioptric partial objective. Because of the use of lenses, it is not possible to use this objective in the case of EUV wavelengths (<20 nm). The projection objective is used, for example, in an inspection system for observing wafer surfaces.